Alternative lighting devices to replace incandescent light bulbs differ from incandescent light bulbs in the manner that energy is converted to light. Incandescent light bulbs include a metal filament. When electricity is applied to the metal filament, the metal filament heats and glows, radiating light into the surrounding area. The metal filament of conventional incandescent light bulbs generally has no specific power requirements. That is, any voltage and any current may be applied to the metal filament because the metal filament is a passive device. Although the voltage and current need to be sufficient to heat the metal filament to a glowing state, any other characteristics of the delivered energy to the metal filament do not affect operation of the incandescent light bulb. Thus, conventional line voltages in most residences and commercial buildings are sufficient for operation of the incandescent bulb. However, incandescent bulbs are not very efficient in conversion of energy to light, and thus waste energy.
One alternative lighting device with better efficiency is a light-emitting diode based bulb (LED-based bulb). The LEDs in the bulb consume energy from a line input source and convert the energy to light through photoemission. However, the LEDs, unlike the metal filament, are not a passive component. Whereas the metal filament presents a nearly constant resistance to the line voltage source and can operate from AC voltages, LEDs are DC devices that need to have a controlled supply current. The controlled supply current is conventionally supplied by one or more power stages placed between the LEDs and the line voltage source. The power stages convert energy from the line voltage source to an appropriate input for the LEDs. The power stages also regulate the conversion of energy from the line voltage to the LEDs by regulating current through the LEDs because the emitted light is proportional to the current.
Line voltage sources are generally alternating current (AC) waveforms. Because the voltage at the line source varies with time, the energy available to the LED-based bulb also varies over time. Without control over the conversion of energy within the LED-based bulb, the light output of the LED-based bulb would ripple over time along with the variations at the line voltage source. One conventional lamp circuit for controlling a LED-based bulb to reduce variations is shown in FIG. 1.
FIG. 1 is a circuit schematic illustrating a conventional two-stage, line-operated lamp circuit. In circuit 100, a line voltage input node 102 is coupled to rectifiers 110, which convert alternating current (AC) at input node 102 to direct current (DC) for output to a first stage DC-DC converter 112. First stage 112 delivers a peak power to capacitor 114 of approximately double the average power consumed by light-emitting diodes (LEDs) 104. A second stage DC-DC converter 116 consumes energy stored in the capacitor 114 and generates a constant current to drive the LEDs 104. Although the circuit 100 provides a constant current with little ripple to the LEDs 104, the circuit 100 includes two power converters, which increases the final cost of an LED-based bulb containing the circuit 100.
An alternate lamp circuit with only a single power converter for performing a power factor correction (PFC) is shown in FIG. 2. FIG. 2 is a circuit schematic illustrating another conventional line-operated lamp circuit with a single stage. A circuit 200 receives line voltage input at node 102, which is provided to rectifier 110. The DC output of the rectifier 110 is provided to capacitor 114 in parallel with transformer 212 and switch 216. The capacitor 114 is a relatively small capacitor, such as 10-500 nanoFarads. The transformer 212 delivers energy from the rectifier 110 to the capacitor 214 and isolates capacitor 214 from the rectifier 110. The capacitor 214 stores energy during peaks in the output of rectifier 110 and discharges energy during the troughs in the output of rectifier 110 to LEDs 104. The LEDs 104 have small resistances to improve the efficiency of conversion of energy to light. Thus, to reduce ripple in the current at LEDs 104, the capacitor 214 must have a large capacitance. The physical size of capacitor 214 increases proportional to capacitance. Thus, the circuit 200 can become costly to manufacture and occupy too much space when capacitor 214 is large. To reduce cost, the capacitor 214 is generally decreased in size, but the size reduction results in larger ripples in current through the LEDs 104, and consequently ripples in the brightness of light output by the LEDs 104. Further, the LEDs 104 have a minimum required voltage, the forward bias voltage, in order to maintain the generation of light. Because the capacitor 214 acts as the energy supply for maintaining the forward bias voltage, the capacitor 214 must be a relatively large capacitor 214. Additionally, other characteristics of the LEDs 104 place requirements on the size of the capacitor 214 in the design of FIG. 2 based on, for example, the non-linearity of the LEDs 104. Although the solution of FIG. 2 resolves the problem of too much ripple in the output light, the circuit 100 is too costly to implement in low-cost devices, such as low-cost consumer light bulbs.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved lamp circuits particularly for LED-based light bulbs. Embodiments described here address certain shortcomings but not necessarily each and every one described here or known in the art.